Twin SSDV Channels with an SDR

With SDRs such as the Funcube and various digital TV dongles, they have enough bandwidth that it can be possible to receive and decode 2 balloon trackers at once. Here’s how, using the Funcube dongle, HDSDR software, VAC (Virtual Audio Cable) and dl-fldigi, all on Windows. The same thing should be possible in a similar manner with other software and operating systems.

Set up 2 Virtual Audio Cables

Set up two virtual audio cables. You will then have 2 VAC audio playback devices and 2 VAC audio recording devices. e.g.:

SDR Software

Open 2 copies of HDSDR. Click the Soundcard button in one copy and set the “Rx Output” device to be the first of your VAC devices:

Now click the Start button in one copy, and select a frequency range for the main waterfall that includes the signals from the payload. Once you can see those, click Start on the second copy of HDSDR. Not that if you later change the frequency range on one copy it will affect both copies.

When you can see the payload signals, select one of those in one copy (click on the waterfall) so that the audio output from that copy relates to that signal. Do the same in the other copy of SDR, for the other payload signal. Here I have the higher frequency signal selected in the upper copy of HDSDR:

Decoding

Now run 2 copies of dl-fldigi. Set the baud rate etc as normal, in both copies. Then on each in turn choose Configure then Sound Card from the menu, choosing the first VAC device in one copy and the second in the other:

Set the baud rate etc in both copies, then align the 2 red lines with the signals in the waterfall as you would usually do:

You are now decoding two telemetry streams with one receiver!

Update: I’m told that some other SDR programs do not allow the one device to be used by multiple copies. This applies to SDR# and gqrx. However another program called SDR Radio does, in V2, allow up to 6 VFOs to be used once, so you could use the same technique as above but with one copy of that program running.